1. Field of the Invention
Example embodiments generally relate to a fixing device and an image forming apparatus, and more particularly, to a fixing device for fixing a toner image on a recording medium and an image forming apparatus including the fixing device.
2. Description of the Related Art
Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having at least one of copying, printing, scanning, and facsimile functions, typically form an image on a recording medium according to image data. Thus, for example, a charger uniformly charges a surface of an image carrier; an optical writer emits a light beam onto the charged surface of the image carrier to form an electrostatic latent image on the image carrier according to the image data; a development device supplies toner to the electrostatic latent image formed on the image carrier to render the electrostatic latent image visible as a toner image; the toner image is directly transferred from the image carrier onto a recording medium or is indirectly transferred from the image carrier onto a recording medium via an intermediate transfer member; a cleaner then collects residual toner not transferred and remaining on the surface of the image carrier after the toner image is transferred from the image carrier onto the recording medium; finally, a fixing device applies heat and pressure to the recording medium bearing the toner image to fix the toner image on the recording medium, thus forming the image on the recording medium.
The fixing device used in such image forming apparatuses may employ a fixing roller heated by a heater and a pressing roller pressed against the fixing roller by a spring to form a fixing nip therebetween through which a recording medium bearing an unfixed toner image is conveyed. As the recording medium passes through the fixing nip, the fixing roller heated by the heater and the pressing roller apply heat and pressure to the recording medium, thus melting and fixing the toner image on the recording medium.
Various types of recording media are available in the fixing device. However, if an envelope is conveyed through the fixing nip under enhanced pressure identical to pressure appropriate for fixing the toner image on plain paper, the envelope may crease. Further, if the pressing roller is pressed against the fixing roller with enhanced pressure constantly, the pressing roller and the fixing roller may suffer from permanent deformation. Moreover, if a recording medium is jammed between the fixing roller and the pressing roller, it may be difficult for the user to remove the jammed recording medium from between the fixing roller and the pressing roller pressed against the fixing roller with enhanced pressure appropriate for fixing the toner image on the recording medium. To address these problems, the fixing device may employ a depressurization mechanism that decreases pressure between the fixing roller and the pressing roller as shown in FIGS. 1 and 2.
FIGS. 1 and 2 illustrate a fixing device 20R incorporating a lever 400 serving as a depressurization mechanism that decreases pressure between a fixing roller 200 and a pressing roller 300. As shown in FIG. 1, the pressing roller 300 supported by a support 500 receives a resilient bias from a biasing member 600 via the support 500 and thereby is pressed against the fixing roller 200 with enhanced pressure therebetween. As the user rotates the lever 400 clockwise in FIG. 2 in the direction of the arrow to press the lever 400 against the support 500 against the resilient bias exerted by the biasing member 600, the pressing roller 300 is pressed against the fixing roller 200 with reduced pressure therebetween.
However, in order to solve the above-described problems, it is necessary to detect pressure between the fixing roller 200 and the pressing roller 300 precisely. For example, the fixing device 20R may employ a sensor that detects the position of the lever 400. When the lever 400 is at the reduced pressure position shown in FIG. 2, it is immovable there while receiving the resilient bias from the biasing member 600. Conversely, when the lever 400 is at the enhanced pressure position shown in FIG. 1, the lever 400 is free from the resilient bias from the biasing member 600 and therefore idly rotatable. Accordingly, the sensor may not detect the enhanced pressure position of the lever 400 precisely, resulting in erroneous detection of pressure between the fixing roller 200 and the pressing roller 300 that may raise the problems described above. If an optical sensor providing a limited detection range of from about 2 mm to about 3 mm is used, erroneous detection may occur frequently.